The invention concerns coextruded elastic composites and structures obtainable thereby.
Elastomeric materials have been long and extensively used in garments, both disposable and reusable. Conventionally, the elastic is stretched and in this stretched condition attached to a substrate. After attachment, the elastic is allowed to relax which will generally cause the substrate to shirr or gather. Elastic was at one time applied by sewing, see, e.g., U.S. Pat. Nos. 3,616,770 (Blyther et al.), 2,509,674 (Cohen), and 22,038. More recently, this procedure has been displaced by the use of adhesive, e.g., U.S. Pat. No. 3,860,003 (Buell). Buell proposed the use of an elastic strand in the leg areas of the disposable diaper. Welding has also been proposed in U.S. Pat. No. 3,560,292 (Butter) although sonic welding is preferred. A pivotal problem with all these attachment methods has been how to keep the elastic in a stretched condition while applying it to the substrate. A solution has been proposed in the use of heat shrink elastomeric materials, e.g., U.S. Pat. No. 3,639,917 (Althouse).
In diapers, for example, elastomeric bands are typically used in the waistband portions such as discussed in U.S. Pat. No. 4,681,580 (Reising et al.), and U.S. Pat. No. 4,710,189 (Lash). Both these patents describe the use of elastomeric materials which have a heat stable and a heat unstable form. The heat unstable form is created by stretching the material when heated around its crystalline or second phase transition temperature followed by a rapid quenching to freeze in the heat unstable extended form. The heat unstable elastomeric film can then be applied to the, e.g., diaper and then heated to its heat stable elastomeric form. This will then result in a desirable shirring or gathering of the waistband of the diaper. A problem with these materials, other than cost, is the fact that the temperature at which the material must be heated to release the heat unstable form is an inherent and essentially unalterable property of the material to be used. This inflexibility can cause problems. First, it is more difficult to engineer the other materials with which the waistband is associated so that they are compatible with the temperature to which the elastomeric member must be heated in order to release the heat unstable form. Frequently this temperature is rather high which can potentially cause significant problems with the adhesive used to attach the elastomeric waistband, or, e.g., the protective back sheet or top sheet of the diaper. Further, once chosen the elastomer choice can constrain the manufacturing process rendering it inflexible to lot variations, market availability and costs of raw materials (particularly elastomer(s)), customer demands, etc.
A problem noted with the application of elastic to a diaper, as proposed in U.S. Pat. No. 3,860,003, resides in the proposed use of a single relatively large denier elastomeric ribbon. This ribbon will concentrate the elastomeric force in a relatively narrow line. This allegedly caused the elastic to pinch and irritate the baby's skin. Proposed solutions to this problem included the use of wider bands of elastic as per U.S. Pat. Nos. 4,352,355 (Mesek et al.) and 4,324,245 (Mesek et al.). Allegedly, this allows the contractive forces to be distributed over a wider area and prevents irritation of the baby's skin. The preferred elastomer proposed in these applications are films of A-B-A block copolymers with a thickness of 0.5 to 5 mils. Problems noted with these films are that they are difficult to handle and must be applied with relatively complicated stretch applicators as per U.S. Pat. Nos. 4,239,578 (Gore), 4,309,236 (Teed), 4,261,782 (Teed), and 4,371,417 (Frick et al.).
An alternative solution to the pinching problem of U.S. Pat. No. 3,860,003 is proposed in the use of multiple strands of relatively small denier elastic, as per U.S. Pat. No. 4,626,305 (Suzuki et al), who describes the use of three to 45 fine rubber strings to elasticize a diaper. However, to keep the bands properly aligned they are preferably fused together. The alleged advantage in this method is that a small number of narrow elastic bands can be stretched at a high ratio to give the same tensile stress that a single equivalent diameter elastic band would yield at a lower stretch ratio. Accordingly, the stress can be distributed over a wider area and less elastic needs to be used (i.e., as the elastic is stretched more when applied). A similar approach is proposed by U.S. Pat. No. 4,642,819 (Ales et al.). However, Ales et al. uses larger denier elastic bands which act as backup elastic seals for each other when or if the diaper is distorted during use. A variation of this approach is proposed in U.S. Pat. No. 4,300,562 (Pieniak). Pieniak uses a series of interconnected elastomeric strands, in a reticulate form. Wider strands are positioned to engage the narrow portion of a tapered surface. This allegedly results in a more even distribution of stress over where the reticulate elastic engages the tapered surface. Although the use of multiple strands of elastic materials has advantages, they are more difficult to incorporate into a garment in a spaced coordinated fashion. Thin elastic strands have a tendency to wander and further present a thin profile making adhesion to the garment substrate difficult.
Spaced elastic elements are provided other than by multiple elastic strands. For example, it has been proposed to provide regionalized elastic in the waistband portion of a disposable diaper in U.S. Pat. No. 4,381,781 (Sciaraffin).
Regionalized elastic is also placed in diaper adhesive fastening tabs as per U.S. Pat. Nos. 4,389,212 (Tritsch), 3,800,796 (Jacob), 4,643,729 (Laplanche), 4,778,701 (Pape) and 4,834,820 (Kondo et al.). These patents are directed to different composite structures designed to yield a fastening tab with an elasticized central portion and inelastic or relatively rigid end portions for attachment to either side of a garment closure. These composites are quite complicated and generally are formed by adhering several separate elements together to provide the elasticized central region.
Elastomers used in these structures also exhibit relatively inflexible stress/strain characteristics which cannot be chosen independently of the activation temperature. Materials with a high modulus of elasticity are uncomfortable for the wearer. Problems with a relatively stiff or high modulus of elasticity material can be exaggerated by the coefficient of friction and necking of the elastomer which can cause the material to bite or grab the wearer.
In copending application Serial No. 07/438,593, filed Nov. 17, 1989, now U.S. Pat. No. 5,501,679 having a common assignee, there is disclosed a multi-layer elastic laminate having at least one elastomeric layer and at least one coextensive skin layer which addresses certain of the above noted problems in the art. In addition, the laminate has extremely useful and novel properties. When cast, or after formation, the elastic laminate is substantially inelastic. Elasticity can be imparted to the inelastic form of the laminate by stretching the laminate, by at least a minimum activation stretch ratio, wherein an elastic laminate material will form immediately, over time or upon the application of heat. The method by which the elastic laminate material is formed can be controlled by a variety of means. After the laminate has been converted to an elastomeric material, there is formed a novel texture in the skin layer(s) that provides significant advantages to the laminate. Despite the numerous advantages in the materials of the copending application, there is room for improvement for some applications such as those discussed above. For example, where intermittent elasticized regions are desired such as in a diaper fastening tab or where it is desirable to have discrete adjacent longitudinal bands of elastic. In these applications, laminated plastic films are less desirable. For example, they must be assembled into complex composite structures as discussed above to provide regionalized elastic. Therefore, it is desirable to retain the advantages of the material disclosed in the copending application while providing structures having regionalized elastic areas or elastic bands which can be simply constructed and are also more resistant to delamination than multi-layer laminate structures.